A rear door heat exchanger (RDHx) is a liquid-based cooling solution installed on the back of a server rack to capture and remove heat from server exhaust. Rather than relying solely on room air conditioning, it transfers heat to a circulating water loop, helping data centers efficiently cool high-density IT equipment while maintaining a conventional rack layout.
Why Are RDHxs Used in Data Centers?
RDHxs are designed to address the rapidly growing cooling demands of high-density server racks, specifically those supporting AI, machine learning, and high-performance computing (HPC) applications. As these workloads produce more heat than traditional IT equipment, conventional air-cooling methods become less effective.
By removing heat directly from the server exhaust at the rack level, RDHxs enable higher rack power densities, improve thermal performance, and help extend the capacity of existing data center cooling infrastructure without requiring a full transition to liquid-cooled servers.
How Does a Rear Door Heat Exchanger Work?
An RDHx is installed on the rear of a server rack, where it intercepts the hot air being exhausted by the IT equipment. As the air passes through the heat exchanger, a closed-loop flow of chilled water absorbs the heat before the air is returned to the data center at a lower temperature. The warmed water is then sent to the facility’s cooling system, where the heat is removed and the water is recirculated. Because the cooling takes place at the rack, an RDHx reduces the amount of heat released into the data center while allowing servers to maintain their normal airflow patterns.
Types of Rear Door Heat Exchangers
Passive RDHx
A passive rear door heat exchanger uses the airflow generated by the server’s internal fans to move hot exhaust air through the heat exchanger. Since it does not require additional fans, it consumes less energy and is a good fit for low-to-medium density server racks where existing air flow is sufficient for cooling.
Active RDHx
An active rear door heat exchanger incorporates built-in fans to increase airflow through the cooling coil. This added airflow improves heat removal, making active RDHxs well suited for high-density environments including AI and HPC deployments, where server-generated airflow alone may not provide enough cooling capacity.
Benefits of Rear Door Heat Exchangers
RDHxs help maintain stable rack temperatures by removing heat before it spreads into the data center environment. They reduce hot spots, decrease the cooling burden on room-level systems, and can often be integrated into existing rack infrastructure. Because they typically require less construction and disruption than direct-to-chip cooling, RDHxs offer a scalable, cost-effective way for organizations to retrofit existing facilities and accommodate higher-density or AI workloads without immediately committing to a full redesign. For the very highest AI rack densities, however, direct-to-chip cooling still offers greater headroom.
Track and Model RDHxs with DCIM Software
Sunbird's Data Center Infrastructure Management (DCIM) software provides asset and relationship management for door heat exchangers and direct-to-chip liquid cooling infrastructure. Operators can create RDHx items, place them in cabinets, and build a complete digital twin of their liquid cooling infrastructure — including CDUs, manifolds, piping, valves, and port-to-port connections.
For RDHx and broader liquid cooling deployments, DCIM software can help:
- Visualize infrastructure. Track, model, and visualize heat exchangers alongside CDUs, manifolds, and piping.
- Plan and manage deployments. Simplify planning for high-density and AI infrastructure rollouts.
- Perform impact analysis. Leverage a digital twin with relationship and dependency modeling to understand how a change to one component affects connected racks and equipment.
- Improve efficiency and uptime. Gain visibility into liquid-cooled compute equipment (e.g., GPUs) and its supporting infrastructure to reduce the risk of overheating and downtime.
Want to see how Sunbird’s leading DCIM solution can help leverage your RDHxs? Get your free test drive now.